Thy-1 is a cell surface differentiation marker expressed predominantly in the brain of many mammalian species including man. In some species, notably rats and mice. Thy-1 is also expressed on lymphoid tissue. Its preferential expression on synapses and its appearance during morphologic development of the brain suggest that it may play a role in synapse formation. The tissue-specific expression of Thy-1 and its variation in patterns of expression among different species make Thy-1 an ideal model for studying the fundamental mechanisms that underlie gene regulation. We propose to identify DNA sequences that regulate the Thy-1 gene in a tissue-specific manner. Recombinant DNA constructs will be used in transient and stable transformation systems. In addition, transgenic mice expressing the human Thy-1 gene will be used to study the mechanisms responsible for the differing patterns of Thy-1 expression in different species.